In exposure control of an exposing device, it is known that frequency of clocks is modulated by a spread spectrum method in order to reduce radiation noises. In such technology, since lighting period depends on the clock, the exposure amount of each pixel differs at parts where frequency of clocks is high and at parts where frequency of clocks is low, and density variation is generated in an image. In a known technology, by setting a scanning cycle for scanning one scanning line and a modulation cycle of clocks so as to have a particular relationship, phases of modulated waveform (change of frequency of clocks) are shifted among a plurality of scanning lines, thereby correcting density variation of an image.